|Publication number||US6983097 B2|
|Application number||US 10/840,846|
|Publication date||Jan 3, 2006|
|Filing date||May 7, 2004|
|Priority date||Feb 11, 2002|
|Also published as||US6816637, US20030152305, US20040208472|
|Publication number||10840846, 840846, US 6983097 B2, US 6983097B2, US-B2-6983097, US6983097 B2, US6983097B2|
|Inventors||Maurice McGlashan-Powell, Philip Charles Danby Hobbs|
|Original Assignee||International Business Machines Corporation|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (5), Referenced by (25), Classifications (16), Legal Events (2)|
|External Links: USPTO, USPTO Assignment, Espacenet|
This application is a division of application Ser. No. 10/073,450, filed Feb. 11, 2002, now U.S. Pat. No. 6,816,637.
This invention relates to a system, and a method, for using an optical backplane to provide ultra high frequency optical interconnections amongst microprocessors. The backplane is comprised of an optical waveguide network and magneto-optical routers.
Th speed of computing devices, such as electronic processors, has been steadily increasing. Processing speed is accompanied by a need for rapid communication amongst processing units. The communication bandwidth requirements of microprocessors (the words processing unit and microprocessor will be used interchangeably) are roughly proportional with speed, and chip cycle times are reaching into the GHz domain. The central computing complexes of large computers comprise of many individual microprocessors, packaged into multi chip modules (MCM), combining their individual performance in a fashion that is transparent to end user. This is only possible if the individual microprocessor chips communicate with each other at sufficiently high bandwidth. Another arena where processor unit to processor unit communication is of concern is the massively parallel computing approach. In such computers hundreds, or thousands, of individual processors, each possibly comprising of several microprocessors, have to be all interconnected at high speeds. A major packaging challenge of computing systems always has been the communication infrastructure, the so called wiring backplane of sufficient bandwidth. The way this is presently done is to have sufficient number of metal wiring in the backplane, connecting the processors to one another. However, as data rates increase metal interconnects between chips, or between multi chip modules, are reaching their limits. Communication between processor chips is starting to be a performance bottleneck. Metal interconnects suffer from loss, cross talk, excessive power requirements, all limiting the maximum achievable bandwidth. As a result of such difficulties optical interconnections are now being seriously considered to take the place of metal wiring.
Optical interconnects have the distinct advantage of almost limitless bandwidth, no cross talk, and low loss. However, the actualization of a purely optical backplane hitherto faced formidable obstacles. There are problems with the integration of lasers, detectors, and waveguides into necessarily small spaces afforded in microprocessor technology. There is also the problem of how to direct light pulses along an optical network at GHz speeds. Then, there is the problem of process integration, namely the difficulty of the processing technology needed to incorporate lasers, detectors, and waveguides into a CMOS technology framework.
The object of this invention is an optical backplane and methods of its use in an electronic processing system. Such a processing system comprises of a large number of microprocessors, with the backplane providing connections amongst the processing units. The processing system can be a single processor in a multi chip embodiment, in which case the processing units are individual chips, or the processing system itself can be a multi processor, in which case the individual processing units again can be chips, or can alternatively be MCMs. Or, one can have combinations of these, depending on the particulars of a system, as one skilled in the art would observe.
It is a further object of the invention to use thin film technology for creating the optical backplane, such a technology being similar and compatible to that used in CMOS technology, whereby such an optical backplane can be integrated with CMOS technology.
It is yet a further object of the invention to provide for routers in such an optical network. These routers are based on magneto-optical polarization rotator and polarization beam splitter combinations.
It is yet a further object of the invention to provide for the whole optical interconnection network, based on planar, ridge, or cylindrical waveguides. Such waveguide types are well known in the appropriate arts. Also, for providing apparatus and method for controlling the routers in such an optical network. The routers allow establishing of communication amongst processing units at a speed commensurate with the bandwidth requirements. Communication amongst processing units can mean interconnecting any two unit, or to allow for a broadcasting mode, where one processing unit simultaneously transfers data to more than one other unit, or possibly to all of the other units.
The light in the network originates when electrical signals from each microprocessor drive an array of lasers, preferably Vertical Cavity Surface Emitting Lasers (VCSEL'S). Laser light is polarized, and when such light is steered into the waveguide network through the optical devices that operationally connect the processing units to the network, it enters the waveguides in a polarized state. The operation of the optical routers is based on the fact the light in the network is polarized. There are several ways, based on polarization beam splitters, to direct light into differing optical paths depending on the polarization angle of the light. If such a polarization beam splitter is preceded by an optical element which is capable to controllably set the polarization angle, one has achieved an optical router which is part of an optical waveguide network. In the preferred embodiment such an optical element, which controllably sets the polarization angle, is a magneto optic rotator (MOR). In a MOR the waveguiding layer has magnetic properties, and depending on its magnetization state it rotates the polarization angle of the guided light. In a preferred embodiment such a magneto-optically active layer comprises of Yttrium Iron Garnet (YIG).
When the polarized light passes through a YIG waveguide segment, the polarization of the incident light can be converted from TE to TM mode (horizontal to vertical polarization), or vice versa, depending on the magnetization within the YIG. In a preferred embodiment this mode conversion is done in two steps, using two sections of YIG material. In the first step the incident polarization is rotated by +45° or −45°. A second section of YIG waveguide has its magnetization permanently aligned parallel to the direction of light propagation and gives a constant +45° of rotation to the incident light, which has already been rotated by +45° or −45°. This then gives a final angle of rotation of 90° or 0°, depending on the choice made in controlling the magnetic field in the first section. One skilled in the art will observe that the operation of the routing scheme is not in need of polarization rotation angles which are exactly of the desired values. There is some latitude of having the polarization rotations accomplished to within a few percent of the exact desired values.
These and other features of the present invention will become apparent from the accompanying detailed description and drawings.
The MOR is also shown in a sandwich structure embodiment. A magneto-optically active layer 110, in a preferred embodiment a YIG layer, being disposed between a gadolinium gallium garnet (GGG) layer 115 and a cover layer 120. The cover layer 120 in a preferred embodiment is another layer of GGG, but it can be made by many other optical materials as long as it has a refractive index below that of YIG. The cover layer 120 can be omitted, with air (or vacuum) taking up the appropriate optical role. The MOR has a magneto-optically active layer which interfaces, that is, it is deposited on the top, or below, of an additional optical layer. The requirement of this additional optical layer is for it to have a lower refractive index than the magneto-optically active layer. The cover layer 120 is a third optical layer making up the sandwich structure together with the magneto-optically active layer 110 and the additional optical layer 115. In case of a sandwich structure the two layers between which the magneto-optically active layer is being disposed are two other optical layers. In order for the light be guided in the magneto-optically active layer, the these two other optical layers have lower refractive indexes than the magneto-optically active layer. The polarized light is guided in the YIG layer 110, which receives the light k 100, from the doped SiO2 layer 130, and transmits the light k 101 back to the doped SiO2 layer 130. The index k being a light propagation wave vector indicating propagation and wavelength.
The YIG layer 110 can be grown by liquid phase epitaxy (LPE) or epitaxial sputter deposition on GGG layers. The index of refraction of GGG layer 115 is approximately 1.94, while that of the YIG layer approximately 2.18.
An external variable magnetic field 180 is applied to the MOR. The external magnetic field 180 lines up the magnetization of the YIG layer 110, and according to the direction of this lineup and the strength of the field the polarization angle of the light 100, is either rotated by approximately 90°, or it is left approximately intact. Accordingly light 101 might have the opposite (horizontal vs vertical, or vice versa), polarization compared to light 100. Light 101 next arrives to a PBS 150. The shown PBS is of the variety which is constructed into the optical waveguide network. The light guided by layer 130 strikes the PBS 150 which in
The grating 155 transmits the light 101 continuing in the straight direction k 102, if light 101 had one (horizontal or vertical) polarization angle, or deflects it into a side branch of the optical network in direction k′ 103, if light 101 had the perpendicular polarization angle (vertical or horizontal). The polarization angle of light 101 was determined by magnetic field 180, thus by controlling field 180 one can choose the path that the light takes in the network.
In the shown case light 100 with a certain polarization 350 is traveling in the waveguide network 310. In the 330 MOR the polarization angle may be rotated to the opposite state (horizontal vs vertical, or vice versa). Upon arriving to the splitter region 340, with the Brewster angle splitter 300, the light is transmitted 103 if its polarization state has been changed 351, or it is deflected 102 if its polarization state remained the same 350. One can adjust the position of the Brewster angle beam splitter to transmit or deflect with opposite polarization than just described. The result in any case is that depending on a polarization rotation which occurred in the MOR 330, the light is controllably routed.
In the shown case light 100 with a certain polarization 350 is traveling in the waveguide network 310. In the 330 MOR the polarization angle may be rotated to the opposite state. Upon arriving to the splitter region 340, with the birefringent prism 301, the light is transmitted 103 if its polarization state has been changed 351, or it is deflected 102 if its polarization state remained the same 350. The birefringent prism 301 can be built into the waveguide to transmit, or deflect, with the opposite polarization than was just described. The result in any case is that depending on a polarization rotation which occurred in the MOR 330, the light is controllably routed.
The Brewster mirror splitter 300, the birefringent prism 301, and the polarization grating splitter 155, shown in
Once the YIG structures are grown they are patterned by etching, or by lithography into properly spaced approximately 3mm sections that would mate 550 and 551 to the SiO2 structures. The SiO2, or possibly polymer, waveguide network itself is deposited or grown on a substrate 510, which can be removed if needed upon completion of the whole structure. The SiO2 waveguide structure can be lithographically patterned to accommodate the sections of YIG waveguides. In this manner the network of optical waveguides, including routers, are seamlessly meshed together into a coplanar configuration.
Many modifications and variations of the present invention are possible in light of the above teachings, and could be apparent for those skilled in the art. The scope of the invention is defined by the appended claims.
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|U.S. Classification||385/147, 385/6, 398/82, 385/14, 385/24, 385/16|
|International Classification||G02B6/28, G02B6/43, G02F1/295, G02B6/35, G02B6/26|
|Cooperative Classification||G02B6/43, G02B6/2746, G02B6/2848|
|European Classification||G02B6/28B8, G02B6/43|
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